My friend had a huge 46 in monitor and I told him he was crazy. He didn't think 2 would be better until he tried it. The appeal is being able to quickly maximize documents or programs to each screen rather than having to deal with all the mouse work of sizing them up properly.
I currently run 2 large 24" monitors in the middle and than 2 19" to the left and right of those (one of the 19" widescreens turned so its long edge is vertical is ideal for reading pdfs and spreadsheets). All that real estate is invaluable when you are neck deep in datasheets, schemtics, layouts, browser windows, emails, powerpoints, excel spreadsheets, matlab documents, and some fea simulation packages. I usually have about 20 to 30 individual things running at a time and wanting to have 4-6 in view at a time.
There are alot of studies out there showing the increased productivity of multiple display setups and alot of companies adopt it as a standard for the engineering workstations.